In the production of chemical cellulose pulp (e.g. paper pulp) it is highly desirable to obtain uniformity of treatment. One important way that this uniformity is typically achieved or approached is to provide uniform impregnation of the cooking liquor (e.g. white liquor) into the comminuted cellulosic raw material (typically wood chips). In order for there to be uniform impregnation the air must be removed from the chips, and this is typically done by steaming.
In approximately the 1970s, it became common to at least initiate steaming of the chips at an early stage in their treatment by supplying steam to a conventional vertical vessel known as a "chip bin". In most systems, chips were fed into the top of the chip bin, e.g. through an air lock, where they were subjected to steam before moving downwardly through the bin into a chip meter, and then a low pressure feeder, subsequently to a horizontal steaming vessel where the removal of air in the chips with steam was completed, and then either a feed mechanism on top of a batch digester, or more commonly to a high pressure feeder for a continuous digester. In addition to providing a volume for initial steaming, the chip bin provides a storage volume sufficient to insure supply of the continuous digester, and/or like components, on a regular basis even though the chips are not continuously fed from a chip heap or pile to the pulping system. This is especially important in winter weather conditions in cold climates, where many pulp mills are located, because of interruptions in an ability to continuously feed chips from a heap or pile to the pulping system due to freezing of the chips in the pile, or other weather related disruptions. Numerous problems of channeling or "rat-holing" are caused by inhomogeneous chip feed. Frozen chips have different flow properties than normal chips, wet different than dry, and sawdust and pin chips different than whole chips.
It has long been known that when wood chips (and like comminuted cellulosic material) funnel downwardly in a chip bin, or similar vessel, to a discharge having a smaller cross-sectional area than the area of the vessel (chip bin) itself there is a tendency for the chips to hang up or bridge. Also some areas allow channelling of the chips to the discharge, while in other areas the chips move little. This is a significant problem because it can interrupt the continuity of supply and thereby defeat a major purpose of a chip bin. Therefore since at least as early as the 1970s conventional chip bins have often included a vibratory discharge mechanism which continuously or periodically shakes the discharge, minimizing bridging and the possibility of plugging, and promoting uniform flow of chips through all portions of the chip bin. One such conventional vibratory discharge is shown in U.S. Pat. No. 4,124,440 and Canadian Patent 1,146,788, both of which also show conventional mechanisms for steaming the chips while in the chip bin.
While vibratory discharges for chip bins have long been the commercially preferred way of preventing bridging, and have long worked well, as the size of pulping systems--and therefore the size of the chip bin associated therewith--has increased in the 1980s and 1990s, there have been increasing practical operational difficulties. In fact for chip bins having a maximum diameter of over about twelve feet (and certainly over fourteen feet) problems in plugging, bridging, and channeling have increased (especially for some woods, such as cedar), as have maintenance and reliability problems associated with the vibratory discharges. Some of these problems can be greatly alleviated or solved by using conical inserts for the chip bin as shown in U.S. Pat. No. 5,454,490, (the disclosure of which is hereby incorporated by reference herein), however even with the system and method described therein maintenance and reliability problems of a vibratory discharge, or other problems, may still occur for chip bins having a maximum diameter of about twelve feet or more.
According to the present invention, a method and apparatus are provided which specifically address the problems of reliability and maintenance of conventional vibratory discharges, and the problems of chip bin pluggage, bridging and/or channelling. While the invention is primarily directed to chip bins having a maximum diameter of about twelve feet or more, many aspects thereof are appropriate for bins in general, and of almost any size. The invention utilizes mass flow (as contrasted with the "funnel flow" of U.S. Pat. No. 5,454,490) in the chip bin, which has significant benefits in promoting uniform steaming, and in minimizing channeling.
According to the invention, the vibratory discharge is replaced with a simpler, less troublesome, more easily maintained structure while not only not sacrificing discharge efficiency and the ability to steam the chips, but actually enhancing them. Also, in some of the embodiments of the invention, the chip meter--a conventional and necessary piece of equipment associated with most chip bins for continuous digester systems--can be eliminated without elimination of its metering function, thereby resulting in the potential for equipment and maintenance savings for the chip feeding system as a whole.
According to the general method of the present invention, comminuted cellulosic material is fed to a digester using a vertical open interior chip bin having a top and bottom, and a maximum diameter of about twelve feet or more (e.g. fourteen feet or more), and a discharge operatively connected to a digester. The discharge has a cross-sectional area much less than half of the cross-sectional area of the chip bin (e.g. less than one-tenth). The method comprises the steps of: (a) Feeding the comminuted cellulosic material into the top of the chip bin, to flow downwardly in a column in the chip bin toward the bottom. (b) Causing the comminuted cellulosic material to move into a gradually restricting open flow path in the open interior of the chip bin having a cross-sectional area less than half of the area at the maximum diameter of chip bin. (c) Without vibrating the chip bin or the chip bin discharge, causing a substantially uniform flow of comminuted cellulosic material in the gradually restricting open flow path, substantially without bridging or hangups of the comminuted cellulosic material in the flow path. (d) Steaming the comminuted cellulosic material while in the chip bin. And, (e) discharging the comminuted cellulosic material from the chip bin discharge and feeding it to the digester.
Step (e) may be practiced by feeding the material directly from the discharge to a low pressure feeder and then ultimately to a digester, or alternatively the material may be fed directly from the discharge to a chip meter, and then ultimately to the digester. Steps (b) and (c) may be practiced by causing the comminuted cellulosic material to flow into two distinct volumes each comprising about half of a main volume defined by a substantially circular cross-section top and a substantially rectangular cross-section bottom, and a larger cross-sectional area at the top thereof than at the bottom thereof, and opposite non-vertical gradually tapering sides, and causing the material to move from each distinct volume to the discharge using oppositely rotating feed screws (or opposite handed feed screws rotated by a common shaft), the discharge being located approximately midway between the two distinct volumes. Steps (b) and (c) may be further practiced by causing the material to flow into distinct volumes wherein the degree of taper of the opposite non-vertical gradually tapering sides is about 20.degree.-35.degree.. Alternatively, steps (b) and (c) may be practiced by causing the comminuted cellulosic material to flow through a transition having one dimensional convergence and side relief between a first volume having a circular cross-section of at least about twelve feet and a discharge having a circular cross-section of much less than half of the first volume.
Step (d) is typically practiced by adding steam to the distinct volumes by introducing the steam into a substantially vertical chip bin wall interruption in at least one non-vertical gradually tapering side of each of the distinct volumes.
According to another aspect of the present invention a bin is provided in general. While the bin has specific utility as a chip bin, particularly for diameters of about twelve feet or more, it is useful for almost any size chip bin, and for other bin constructions in general. According to this aspect of the invention the bin comprises: A hollow substantially right circular cylindrical main body portion having a substantially vertical central axis, a top and an open bottom. A top wall closing off the top of the main body portion, and having means for introducing particulate material into the hollow main body portion mounted thereon. A hollow transition portion connected to the bottom of the main body portion having a substantially circular cross-section open top and a substantially rectangular cross-section open bottom, and a larger cross-sectional area at the top thereof than at the bottom thereof, and opposite non-vertical gradually tapering side walls. At least one feed screw mounted adjacent the open bottom of the transition portion, in a housing. A discharge operatively connected to the feed screw housing. And, means for rotating the at least one feed screw to move particulate material from the bottom of the transition portion to the discharge.
The bin may further comprise means for introducing steam to the hollow transition portion, the means comprising a steam conduit, and a substantially vertical wall interruption of at least one of the non-vertical gradually tapering side walls of the transition portion, the steam conduit connected to the substantially vertical wall interruption. The non-vertical gradually tapering side walls of the transition portion may each have a degree of taper that is about 20.degree.-35.degree. (typically about 25.degree.-30.degree.) with respect to vertical, which is about 10.degree.-20.degree. greater than the mass flow angle for the material handled (the mass flow angle for most chips is about 10.degree.-15.degree.).
The at least one feed screw may comprise first and second feed screws mounted at the bottom of the transition portion, a junction provided between the screws, and each mounted for rotation about a common generally horizontal axis; and the means for rotating the at least one feed screw may comprise means for rotating the first and second screws about the axis in different directions (or opposite handed feed screws rotated by a common shaft). The structure also preferably includes a baffle disposed within the transition portion above the screw junction; and, the discharge may comprise a substantially right rectangular parallelepiped discharge operatively mounted to the screws substantially at the screw junction and remote from the transition portion, the discharge for receipt of particulate solid material from both the screws.
Alternatively the discharge may be offset from the main body portion in which case the at least one screw comprises a single screw that transports particulate material substantially horizontally in a single direction from the transition portion to the offset discharge.
As another embodiment, the at least one screw comprises first and second screws, one mounted above the other for rotation about parallel axes, the first screw having a housing mounted to the transition portion and having an outlet therefrom offset from the main body portion, and the second screw having a housing with an inlet connected to the first screw housing outlet, and having the discharge as the outlet, the discharge being substantially concentric with the main body portion. In this case the means for rotating the at least one screw comprises means for rotating the first and second screws so that they transport particulate material in opposite substantially horizontal directions.
According to yet another modification, the transition portion comprises a first transition portion, and further comprises a second hollow transition portion between the first transition portion and the at least one screw, the second transition comprising a hollow substantially right triangular prism with an open top and open bottom and having a larger cross-sectional area at the bottom than at the top, and the cross-sectional area of the top being approximately the same as the cross-sectional area of the bottom of the first transition portion. The bottom of the second transition portion has a length at least five times its width; and the discharge from the screw trough is a rectangular in cross section, having a diameter approximately equal to the width of the bottom of the second transition portion, and is substantially concentric with the main body portion.
According to a still further embodiment the at least one feed screw comprises first and second feed screws mounted at the bottom of the transition portion, a junction provided between the screws and each mounted for rotation about a common generally horizontal axis. The means for rotating the feed screw comprises means for rotating the first and second screws about the axis in different directions. The discharge from the screw trough may comprise a substantially right rectangular parallelepiped discharge operatively mounted to the screw substantially at the screw junction and remote from the transition portion, the discharge for receipt of particulate solid material from both the screws. An agitator may also be provided at the screw junction, and a chip meter, operated by a motor, may be connected to the discharge. A controller coordinates the operation of the chip meter motor and the means for rotating the first and second screws.
According to another aspect of the present invention a chip bin assembly is provided comprising the following elements: A hollow substantially right circular cylindrical main body portion having a substantially vertical central axis, a top and a bottom, and having a first diameter. A top wall closing off the top of the main body portion, and having means for introducing wood chips into the hollow main body portion mounted thereon. A hollow substantially right rectangular parallelepiped discharge having a second diameter with is less than one half of the first diameter. A hollow transition portion disposed between the main body portion and the discharge having one dimensional convergence and side relief. Means for introducing steam to the hollow interior of the bin. And, means for connecting the discharge to a digester.
The assembly may also comprise first and second feed screws mounted adjacent the bottom of the transition portion, a junction provided between the screws, and each mounted for rotation about a common generally horizontal axis: and means for rotating the screws about the axis in different directions (or opposite handed feed screws rotated by a common shaft) to move wood chips from the transition portion to the discharge conduit. Alternatively the transition portion may include at least one substantially planar non-vertical wall portion; and the means for introducing steam into the bin preferably introduces steam into the transition portion, and comprises a steam conduit, and a substantially vertical wall interruption of the substantially planar non-vertical wall portion of the transition portion, the steam conduit connected to the substantially vertical wall interruption.
It is the primary object of the present invention to provide for the effective feeding of particulate material, such as wood chips, downwardly in a bin without the necessity of a vibratory discharge, even where the diameter of the bin is twelve feet or more. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.